1. Field of the Invention
This invention relates to a power steering for applying an assist force using an electric motor, and particularly, to a torque detector for detecting steering torque supplied from a steering wheel.
2. Description of Related Art
Torque detectors for a power steering include, for example, the torque detector disclosed in Japanese Patent Laid-open No. 3-74258 by the present applicant.
In the torque detector, as shown in FIG. 12 and FIG. 13, an output shaft 2 is rotatably mounted in a gear case 1 and has an end at which a pinion 3 is provided. The pinion 3 is engaged with a rack shaft 4 having both ends linking with wheels (not shown).
The rack shaft 4 is linked with an electric motor (not shown) for applying an assist force.
In the gear case 1, an input shaft 5 linking with the steering wheel (not shown) is rotatably mounted coaxially with the output shaft 2. A torsion bar 6 is provided in a hollow portion 5a of the input shaft 5, and has one end secured to the input shaft 5 and the other end secured to the output shaft 2.
In the detector constructed as above, upon turning the steering wheel (not shown), the torsion bar 6 is twisted in accordance with the input torque of the turned steering wheel, to cause a relative rotation between the input shaft 5 and the output shaft 2.
A sensor 18 detailed later detects the amount of relative rotation and the direction of the input and output shafts, and a controller (not shown) controls output of the electric motor based on the detected amount and direction.
The electric motor thus controlled by the controller exerts a driving force in accordance with the input torque on the rack shaft 4 to drive the rack shaft 4 for application of an optimum assist force.
Next, the configuration for detecting the input torque will be explained.
A flange 7 is provided on the input shaft 5. In the flange 7, three planetary gears 8 are arranged at regular intervals along the circumferential direction with respective pins 9 as shown in FIG. 13. The planetary gears 8 engage with a sun gear 10 rotatably provided on the outer circumferential face of the input shaft 5 and a rotatable ring gear 11 rotatably mounted in the gear case 1.
Another flange 12 is provided on the output shaft 2, and three planetary gears (not shown) are also arranged at regular intervals along the circumferential direction with respective pins. The planetary gears engage with the sun gear 10 and a fixed ring gear 13 secured in the gear case 1.
In the flange 7 provided on the input shaft 5, two notches 14 are provided as shown in FIG. 13. Protrusions 15 are provided in the flange 12 on the output shaft 2, and respectively inserted into the notches 14.
A clearance 16 is provided between each notch 14 and the corresponding protrusion 15 to allow the input shaft 5 to rotate relative to the output shaft 2 in the circumferential direction. When the notch 14 and the protrusion 15 make contact with each other, the input shaft 5 and the output shaft 2 integrally rotate. This prevents the torsion bar 6 from becoming extremely twisted and thereby damaged.
A magnetic piece 17 is incorporated in a portion of the outer circumference of the aforementioned rotatable ring gear 11. The sensor 18 made up of a magnetic resistor element is provided on the inner surface of the gear case 1 facing the magnetic piece 17.
The magnetic resistor element has a facility in that it varies an electric resistance therein upon reception of external action of magnetic field and detects the resistance variation as a variation of voltage.
The sensor 18 made up of such a magnetic resistor element detects a voltage in accordance with the magnetic flux variation when the magnetic piece 17 is moved by the rotation of the rotatable ring gear 11. The sensor 18 inputs the detected voltage to the controller (not shown) and the controller controls output of the electric motor. The electric motor exerts the assist force in response to signals from the controller to reduce a steering force to an optimal force.
Next, the operation of the conventional torque detector will be explained.
In the state that a load is transmitted from the wheels (not shown) to the rack shaft 4, when the steering wheel (not shown) is turned, the turning force is transmitted to the input shaft 5. However, since the load from the wheels interferes with the rotation of the output shaft 2, the torsion bar 6 is twisted by the rotation of the input shaft 5. Thus, the input shaft 5 and the output shaft 2 are relatively rotated within the range of the clearance 16.
When the input shaft 5 and the output shaft 2 relatively rotate in this way, the rotatable ring gear 11 rotates as follows.
The limitation of the rotation of the output shaft 2 does not allow the planetary gears (not shown) linked with the output shaft 2 to revolve around the sun gear 10. Additionally, the planetary gears (not shown) cannot rotate due to the engagement with the fixed ring gear 13. For those reasons, the sun gear 10 engaging with the planetary gears (not shown) is under the condition that its rotation is limited.
Under such condition, when the input shaft 5 rotates in relation to the output shaft 2, the planetary gears 8 provided in the flange 7 on the input shaft 5 rotate and revolve around the sun gear 10. Therefore, the rotatable ring gear 11 engaging with the planetary gears 8 rotates slightly.
Upon the rotatable ring gear 11 slightly rotating as explained above, the magnetic piece 17 secured in the ring gear 11 also travels. The sensor 18 detects the amount of travel of the magnetic piece 17, and the controller (not shown) controls the electric motor in response to the detected signal. In this way, the electric motor applies an optimum assisting force.
Since the sensor 18 and the magnetic piece 17 are out of contact with each other as explained above, there is no significant influence of wear and the like in use of a contact type sensor. This allows the detection of high accurate values.
On the other hand, when the input shaft 5 and the output shaft 2 integrally rotate while the torsion bar 6 is twisted, the rotatable ring gear 11 is adapted to stop rotating. This situation will be detailed below.
When the input shaft 5 and the output shaft 2 integrally rotate, the planetary gears (not shown) provided on the output shaft 2 side rotate and revolve around the sun gear 10. Therefore, the sun gear 10 engaging with the planetary gears rotates in the same direction as that of the input shaft 5 and output shaft 2.
The planetary gears 8 provided on the input shaft 5 side revolve in the same direction as that of the input shaft 5 and output shaft 2, while being rotated by the rotation of the sun gear 10. That is to say, the planetary gears 8 rotate while revolving around the sun gear 10.
Such planetary gears 8 travel on the inner face of the rotatable ring gear 11 while engaging therewith due to their rotation. The speed of travel of the planetary gear 8 on the rotatable ring gear 11 is set to be equal to the speed of voluntary revolution of the planetary gear 8 around the sun gear 10. In other words, the planetary gears 8 travel on the inner face of the rotatable ring gear 11 with simply engaging therewith such that the rotating force of the planetary gears 8 does not act on the rotatable ring gear 11. Naturally, the rotatable ring gear 11 does not rotate as long as the rotating force of the planetary gears 8 does not act thereon.
The reason for limiting the rotation of the rotatable ring gear 11 as described above is in order to prevent the magnetic piece 17, provided in the rotatable ring gear 11, from departing from the position facing the sensor 18 even in the integral rotation of the input shaft 5 and the output shaft 2.
Specifically, the magnetic piece 17 is provided only in a portion of the outer circumference of the rotatable ring gear 11. Therefore, if the rotatable ring gear 11 rotates integrally with the input shaft 5 and output shaft 2, the magnetic piece 17 naturally departs from the position facing the sensor 18. In this event, there is the disadvantage that a displacement of the rotatable ring gear 11 is impossible to detect.
For this reason, the conventional detector establishes limitation for the rotation of the rotatable ring gear 11 in order that the magnetic piece 17 faces the sensor 18 at all times.
In the above conventional detector, in order to prevent the magnetic piece 17 provided in the rotatable ring gear 11 from departing from the position facing the sensor 18 even when the input shaft 5 and the output shaft 2 integrally rotate, it is needed to provide the planetary gear unit composed of the planetary gears 8, the rotatable ring gear 11 and so on.
However, providing such a planetary gear unit naturally produces a disadvantage of increasing in size of the gear case 1 housing the planetary gear unit.
Further, the power steering device including such torque detector must be placed typically in a very small space between the foot of the driver seat and the engine room. Therefore, if the gear case 1 is increased in size as explained above, this produces a disadvantage in which the detector cannot be mounted depending on the car models or vehicle types.
It therefore is an object of the present invention to provide a compact torque detector.
A first invention is a torque detector for a power steering, which detects input torque from the amount of relative rotation of an input shaft, linking with a steering wheel, and an output shaft linking with wheels, and supplies the detected signal to a controller, characterized by including: a conversion member provided on the outer circumferences of the input shaft and the output shaft for convert the amount of relative rotation of the input and output shafts to a displacement in the axis direction; an annular-shaped ring member formed of a magnet and secured on the outer circumference of the conversion member; and a plurality of magnetically sensitive devices arranged at intervals around the ring member to oppose each other, in which the magnetically sensitive device detects the amount of travel of the ring member in the axis direction and supplying the detected amount as a voltage signal to the controller.
It should be noted that the magnet in the present invention refers to a magnet generating a magnet flux.
According to the first invention, the conversion member traveling in the axis direction in accordance with the amount of relative rotation of the input shaft and the output shaft is provided with the annular-shaped ring member made up of a magnet. The amount of travel of the ring member in the axis direction is detected by the magnetically sensitive device. The reason for such a configuration is that even when the input shaft and the output shaft integrally rotate, the ring member faces the magnetically sensitive device at all times. Hence, torque can be detected without using the planetary gear unit conventionally, resulting in the reduction in size of the torque detector equal to the space conventionally used for the planetary gear unit.
A second invention is characterized in that the magnetically sensitive devices are connected to a fail detecting mechanism, and the fail detecting mechanism detects a difference between the voltage signals supplied from the magnetically sensitive devices and supplies a fault signal to the controller when the difference exceeds a set value.
According to the second embodiment, when a false signal is input to the controller C because of a failure of the magnetically sensitive device or the like, the fail detecting mechanism supplies a fault signal to the controller, and then the controller stops the electric motor.
Hence, the electric motor does not exert an unexpected assist force.
A third invention is characterized in that a plurality of the magnetically sensitive devices are arranged around the ring member, and the controller averages the detected signals supplied from the magnetically sensitive devices.
According to the third invention, since an average of the detected values supplied from a plurality of the magnetically sensitive devices is found, it is possible to accomplish exact detection of the input torque.
A fourth invention is characterized in that a plurality of the magnetically sensitive devices are arranged at regular intervals in the circumferential direction.
According to the fourth invention, a plurality of the magnetically sensitive devices provided around the ring member are arranged at regular intervals in the circumferential direction. This allows the detection of balanced information, resulting in detection of the input torque with further improved precision.
A fifth invention is characterized in that the conversion member is a tubular member.
According to the fifth invention, since the conversion member is the tubular member that requires a small mounting space, further reduction in size of the torque detector can be accomplished.
A sixth invention is characterized in that the ring member is secured on the tubular member made of metal and a non-magnetic substance is interposed between the tubular member and the ring member.
According to the sixth invention, by interposing the non-magnetic substance between the ring member and the tubular member, it becomes possible to make the tubular member of robust metal.
A seventh invention is a torque detector for a power steering, which detects input torque from the amount of relative rotation of an input shaft linking with a steering wheel and an output shaft linking with wheels and supplies the detected signal to a controller, characterized by including: a conversion member made of up a magnet and provided on the outer circumferences of the input shaft and the output shaft for convert the amount of relative rotation of the input and output shafts to a displacement in the axis direction; a plurality of magnetically sensitive devices arranged at intervals around the conversion member to opposite each other; and a fail detecting mechanism connected to the magnetically sensitive devices, in which the magnetically sensitive device detects the amount of travel of the conversion member in the axis direction and supplying the detected amount as a voltage signal to the controller, while the fail detecting mechanism detect detects a difference between the voltage signals supplied from the magnetically sensitive devices and supplies a fault signal to the controller when the detected difference exceeds a set value.
According to the seventh invention, since the conversion member itself is made up of a magnet, it is unnecessary to additionally provide a ring member to the conversion member. Omitting the ring member enables the reduction of costs.
It should be mentioned that in the sixth invention, the fail detecting mechanism is provided. Therefore, the electric motor does not exert an unexpected assist force.
Since the conversion member is different in shape from those of the ring member, the magnetic flux is also different. However, the controller for detecting the magnetic flux can correct the difference. Accordingly, even in use of the conversion member itself made up of a magnet, precise torque can be detected.
An eighth invention is characterized by further including an adder connected between the magnetically sensitive device and the controller to correct a deviation of an output signal from the magnetically sensitive device in a neutral state.
According to the eighth invention, even when a signal supplied from the magnetically sensitive device in the neutral state is deviated, the deviation can be readily corrected by the adder.